1. Field of Invention
This invention relates generally to the fabrication of high-frequency (or pulsed), high-voltage connectors and, more particularly, to modification of coaxial cable ends to serve as connectors to unite cables with electrical fittings. Disclosed are a method for making the connectors, molds for use with the method, and the connectors, themselves.
2. Description of the Related Art
In a field of high-voltage connectors, it is frequently necessary to adjoin coaxial cables consisting of a central electrical conductor surrounded by a nonconducting or poorly conducting dielectric and an outer conductor which surrounds the dielectric. Ordinarily, the dielectric permits the conducting core of the cable to be insulated electrically from the electric potential of the outer conductor. In high-voltage environments, including in applications involving pulse power, there is a significant risk that when two conductors of opposite respective polarities are placed in sufficiently close proximity to each other, electromagnetic breakdown will occur in the air or other insulating dielectric medium between the conductors resulting in a spark or electromagnetic arc, thus allowing the difference in potential to be neutralized. Thermoplastics, such as high-density polyethylene (HDPE) are common and effective dielectrics for uses associated with high-voltage pulsed power.
One purpose of the dielectric insulator surrounding the conductor material is to provide insulation capable of preventing sparking or arcing. Another purpose is to furnish proper characteristics of impedance determined by the specific composition of the dielectric material, its inner diameter and its outer diameter. Relationships between impedance, dielectric constants and the dimensions (including thickness) of particular dielectrics are well known to those skilled in the art of high voltage connections. It is sufficient to mention here only that it is important to maintain the integrity of the characteristic coaxial impedance of high-voltage cable when splicing cable pieces together or joining cables to fixtures.
The insulating effectiveness of cable dielectric can be seriously compromised where cables are joined to one another or to components. This is due, in part, to the fact that, typically, such connections are not airtight, and gas-filled discontinuities in cable dielectric provide favorable conditions for arcing. This is due to the propensity of air, or other gases, in the presence of high voltages, to break down and allow discharge to occur.
Another concern related to arcing may involve gas bubbles in the dielectric of high-voltage cable. The presence of bubbles in the cable dielectric can increase the risk of an are, much in the same way that a continuous air path across a junction can provide conditions for a spark, although to a lesser extent. In high-voltage applications, it is critical that gas bubbles or other discontinuities in the dielectric insulation be minimized or eliminated. Effective methods for fabricating safe connectors, therefore, cannot permit the continuity and integrity of the cable dielectric to be compromised.
The most commonly used approach to avoid arcing in proximity to high-voltage with a "socket" bearing an opposite configuration. The taper provides for a relatively longer distance, between the central metal conductor and the region outside of the dielectric insulator than a connection where an untapered cable end abuts another untapered cable or fitting. This increased distance relative, for example, to the distance of the radius of the dielectric, allows for a decreased likelihood of discharge due to breakdown of the medium filling the space between the two cable dielectrics. This tapering method has been demonstrated to be an effective connecting technique, as the path length in air between the dielectric surfaces is effectively lengthened. George L. Ragan, Microwave Transmission Circuits, McGraw-Hill Book Company, Inc., 1948, p. 260.
Fabrication of the tapered end is typically accomplished by cutting away the dielectric cable housing using a tool similar to a large pencil sharpener. Although this method has long been an accepted and preferred technique, certain drawbacks exist. For example, the sharpening technique can cause tearing of the dielectric in the region where it adjoins the inner conducting cable, thereby potentially causing gas pockets between the conductor and the dielectric in a location where the electric field is highly concentrated. Also, the sharpening method commonly used often results in a slightly oval cut, which is especially noticeable at the large end of the taper. This can cause difficulty later in securely fastening the tapered end of the dielectric cable into the receptacle of a fitting. Furthermore, it can potentially cause gas pockets to occur between the dielectric of the fitting and that of the connector, thereby increasing the risk of electromagnetic breakdown.
A different approach to minimizing arcing risk in high-voltage connections is to encase the connections in high-pressure gas-filled containers. This disclosure is primarily concerned with dielectrics such as HDPE and other thermoplastics, however, it is instructive to mention also that gases such as nitrogen, given favorable condition of pressure and temperature, can act as efficient dielectric insulators. In high-pressure, high-voltage connections, the high gas pressure in the region of the cable juncture increases gas density in the location of the cable dielectric discontinuity, and the effective amount of gas which would have to undergo electromagnetic breakdown in order for a spark to occur is increased.
In either case, it is necessary and desirable to be able to modify cable ends for high-voltage connections so that they can satisfactorily be mated to fittings used for adjoining the cables to other cables or to fixtures. This includes having the capability to make coaxial cable dielectrics conform to particular shapes favorable for use as cable connectors. Disclosed here is a new method for fabricating cable connectors capable of minimizing the risk of arcing in high-voltage, and in some cases high-pressure, environments. Also disclosed are molds used in the method, and connectors fabricated using the invention method.